When Inspector Yang lands on the red sports car driven by Kevin she breaks the windshield removing the top part of the windshield frame and rearview mirror. In the next shots the frame and rearview mirror are back in place.
The windscreen frame that breaks is a clearly-visible dummy breakaway fitted in place of the Midget Mk II's normal frame; it is taller than the real windscreen.
Also, the Midget Mk II was fitted with triplex-type safety glass which has a middle layer of clear sealant and would break but not fly into pieces, and the early Midgets were fitted with light-alloy windscreen pillars that would most likely snap near the base under that sort of impact.
When the copter carries Kevin over the locomotive's stack, soot from the black smoke smudges the back of his light-coloured trousers. In later shots, the soot is not there.
Also, the stack gasses from an operating locomotive would be so hot that he would probably be badly burnt/scalded.
When Kevin catches the rope ladder he grabs it only a few rungs below the copter, but in later shots, he is hanging on to it almost at the bottom.
During the mining cart escape scene, when the guards reverse the cart Kevin and Panther are in there is another adjacent cart going up the slope. After the camera cuts away and back again the cart has disappeared. The next time the camera cuts away and back again the cart reappears at the top of the slope.
Even if only idling, real helicopter rotor blades would probably shatter, rather than bending if they hit a solid object.
(at around 45 mins) A character is holding a Sega Genesis controller and playing a video game with fighting sound effects. Later in the same scene, Tetris is visible on the screen. Tetris neither produces fighting sound effects nor is even available on the Genesis.
Plywood laid across the gap between two train cars to extend the landing area for the motorcycle jump.